Field of the Disclosure
The present invention relates to a method for treating and/or-pre-treating liquid manure or biogas plant reject for the elimination of harmful substances, particularly nitrogen, phosphorus, and odor molecules.
Description of Related Art
As a consequence of the general intensification of agriculture, the increased farm sizes, the concentration of farming in specific regions and, on the other hand, the specialization of farms in a given line of production, have resulted in a situation where more nutrients are produced than can be spread out on the fields. The situation has culminated in centralized biogas facilities, in which the digestion process is supplied with nutrient-rich feed materials from outside the farm. Nutrients originating from agriculture have been found to constitute the main source of eutrophication in waterways. The runoff of nitrogen is the most important environmental hazard caused by nitrogen losses. The nitrate resulting from nitrification has a particular propensity to runoff. The nitrogen load of a water system causes the overgrowth of nitrogen-limited algae and thereby deteriorates the quality of water. The runoff of nitrogen into potable water reserves may cause health risks in case the nitrogen content is excessively high.
In a particular effort to reduce nutrient runoffs to natural waters, the nitrogen fertilization has been limited by legislation. The nitrates directive issued by the European Union restricts the amount of nitrogen allowed for spreading on the fields. Therefore, some of the farms are forced to remove nutrients further away or completely outside the farm's boundaries. However, most of the liquid manure volume consists of water. This leads to substantial transport and spreading costs. Along with the quantitative production, there are also other problems relevant to using the nitrogen of liquid manure as a fertilizer. The fertilization value of liquid manure nitrogen is difficult to estimate as some of the liquid manure nitrogen is in an organic state. Organic nitrogen is not in the form directly useful for plants, but it is only gradually released into an inorganic form that plants are able to use. On the other hand, some of the liquid manure nitrogen may be lost as gaseous nitrogen emissions, either during storage, at the moment of spreading or immediately after spreading as ammonia gas emissions or by denitrification as nitrogen oxides and/or a nitrogen gas. Hence, the fertilization value of liquid manure nitrogen fluctuates more than that of mineral fertilizers. Because the nitrogen fertilization effect is difficult to estimate, this easily leads to the fertilization being over- or undervalued. In light of the foregoing reasons, there is a demand for methods useful for I) concentrating or separating nitrogen from liquid fraction II) converting liquid manure nitrogen into a form that enables utilizing the fertilization value of liquid manure nitrogen as effectively as possible and thereby reduces the need of using mineral fertilizers.
With regard to the phosphorus of liquid manure, there are several viable methods for reducing its content or its total separation. The phosphorus content can be effectively reduced simply by separating solids and liquid from each other, for example through mechanical separation, whereby a considerable portion of phosphorus remains with the solid fraction. On the other hand, phosphorus can be effectively precipitated from liquid manure chemically, either for ammonium magnesium phosphate (struvite), in which case the nutrients included in the precipitate are in a form useful for plants, or by a method generally used in sewage treatment facilities by precipitating with a trivalent iron or aluminum salt (Fe3+, Al3+). Nitrogen is considerably more challenging than phosphorus in terms of concentrating or separating. In mechanical separation, most of the nitrogen remains in the liquid fraction. In practice, bonding chemically by using a reagent is difficult and just some of the nitrogen can be separated. Separation of the liquid manure ammonia on a countercurrent tower principle (ammonia stripping) has not been implementable thus far in an economically viable manner.
Nitrogen in Liquid Manure
Liquid manure contains nitrogen both in an organic and inorganic form in high concentrations. The average content of soluble nitrogen in liquid swine manure is 2.5 kg/ton, the average total nitrogen content being 3.8 kg/ton. Liquid bovine manure has a total nitrogen content of 3.0 kg/ton, the content of soluble nitrogen being 1.8 kg/ton (Soil Analysis Service statistics). The difference between total nitrogen and soluble nitrogen represents the amount of organic nitrogen contained in liquid manure. Most of the nitrogen has its source in urine and hydrolyzes enzymatically into ammonium within just a few first days from the beginning of storage. For the most part, the soluble nitrogen contained in liquid manure consists of ammonium. With respect to waste water, the nitrogen content of liquid manure is approximately tenfold.
Methods Used for Separating Liquid Manure Nitrogen
The separation of liquid manure nitrogen has resulted in developing and testing a multitude of various methods. These include, among others, various chemical precipitation methods, ion exchange, various membrane filtration methods, biological separation methods, as well as separation of ammonia on a countercurrent tower principle (ammonia stripping). What is typical for most of the methods is the inadequacy to achieve an acceptable standard regarding the separation of ammonia and the economically fairly high price.
Separation of Solids and Liquid
Physical separation has been found to have but a slight effect on a nitrogen fraction dissolved in liquid.
Settling and Mechanical Separation
In mechanical separation, most of the nitrogen remains within the liquid fraction.
Biological Methods
Regarding biological methods, the method that is best known and in common use, particularly as an effluent nitrogen separation method, is a nitrification/denitrification process. Nitrification-denitrification is a two-stage oxidation-reduction process. There, nitrogen is first allowed to oxidize into a nitrate form, i.e. to nitrify. Then, there is needed an oxygen-free denitrification stage, in which the nitrate nitrogen reduces into a nitrogen gas. The method requires precise optimum conditions in order to work.
Other biological nitrogen separation methods have also been developed. As for these, the anaerobic ammonium oxidation reaction (anammox), still under development, has perhaps drawn most attention. The method comprises the oxidation of ammonium directly into a nitrogen gas (N2) while NO2 functions as an electron acceptor. The method is only in an ongoing development stage.
A problem with all biological nitrogen separation methods are fairly high investment and maintenance costs. In the methods, nitrogen is not reclaimed but released as a gas in the atmosphere.
Filtration
Although filtration has been used mainly for the retention of solids, the chemical and possibly also biological properties of a filtered solution are also influenced by filtration. However, the reductions of nitrogen compounds present in a form dissolved in liquid have not been higher than 10%, even at their best.
Reverse Osmosis
Reverse osmosis is a pressure-based membrane purification technique. The principle is based on the use of pressure for forcing a solvent (usually pure water) to pass through a semipermeable membrane from a solution of higher concentration towards one of lower concentration, i.e. the opposite way with respect to osmosis. Making the method applicable to liquid manure nitrogen separation requires a pretreatment of liquid manure (sedimentation, precipitation, microfiltration).
Electrochemical Methods
Electroflotation is a separation method, wherein light particles rise to the surface of wastewater along with small gas bubbles. In this method, the surfaces of electrodes (anode and cathode) develop by way of electrolysis of water small hydrogen and oxygen gas bubbles (22-50 micrometers in diameter). The bubbles rise to the surface of liquid and function at the same time as collectors of finely divided small particles. Electrolysis calls for a powerful electric current. The process is limited by the high dry matter content of liquid manure. In order to enable also for an effective removal of soluble nitrogen, the method requires simultaneous electrochemical oxidation.
Electrochemical Oxidation
There are a variety of ways to carry out electrochemical oxidation. A well-known/common method is the use of chlorine and at-anode formed hypochlorite for the oxidation of ammonia. In the method, the decomposition of ammonium occurs by way of an indirect oxidation reaction. The decomposition takes place through the intermediary of powerful oxidants generated in a liquid solution in an electrochemical reaction. In the presence of chloride, there is chlorine gas generated at the anode. The anode reaction is followed by a diffusion of chlorine gas in the liquid solution (dissolution) and further by a protolyzation into hypochlorite and hypochlorous acid, depending on pH. The method has its effectiveness depending on a salt to be added and the strength of electric current. The effective oxidation of ammonia requires typically a chloride dose of 30 g/L. The problem is a possible formation of chlorinated organic intermediates.
Nitrogen Separation by Stripping
The method comprises reducing the content of ammonia in a liquid phase by passing it in a tower into a gas phase. The method is based on raising first the pH of liquid manure for converting the ammonium nitrogen contained therein into ammonia. This is followed by driving the liquid manure through a tower filled with a packing material promoting the evaporation of NH3, such that the liquid manure is fed into the tower by way of its top end while blowing air into the tower from its bottom part, this resulting in the desorption of ammonia, i.e. its passage into a gas phase from the (boundary) surface of slurry trickling downwards in the packing material. The ammonia gas separated into a gas phase is further passed into water or acid, in which the ammonia gas adsorbs into liquid.
The evaporation of ammonium depends on pH and temperature of the solution. With a rising pH, the ammonium ion dissociates and forms hydrogen ions and ammonia. The resulting hydrogen ions bond with negative OH ions to produce water. [NH4++OH−NH3+H2O]. This equilibrium depends on pH and temperature. In a neutral solution (pH 7), the amount of ammonia molecules in proportion to the total amount of ammonium ions and ammonia molecules is only 0.39%, while that proportion at pH 9 is 28.4% at the temperature of 20 degrees.
Ammonia is very volatile, but also highly soluble in water. According to Henry's law, at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. Thus, it is a consequence of the Henry's law that the partial pressure of ammonia gas on the surface of a slurry solution is directly proportional to the molar ratio of ammonia in the slurry.
Stripping is carried out by using a tower or a tower which is packed with a material with a capability of attaining a specific surface area as large as possible while generating an air resistance as low as possible. As a result, the desorption, i.e. the passage of ammonia from a boundary surface into a gas phase, is achieved effectively. In the tower, air and water are flowing in countercurrent relative to each other, whereby, as the water is flowing down along the surface of a packing material in the form of a film, the air flows upward in the form of a continuous phase. The air flow must be sufficiently powerful and turbulent in order to prevent the air in the proximity of the packing material surface from becoming concentrated with ammonia gas and to provide an effective removal of ammonia. A turbulence of the liquid flow would also be welcome.
A stripping tower has been tested for the separation of liquid manure nitrogen, but this has involved problems because of which the method has not gained the status of general practice, with the exception of research experiments. The reasons can be broken down as follows:
1) The high dry matter content of liquid manure causes clogging of the tower and hampers the progress of stripping; 2) Stripping requires a sufficiently high pH value for the solution to enable a separation of ammonium in the tower as ammonia gas. The untreated liquid manure has a PH of about 7.4. At this pH, the amount of ammonia molecules is only about 1% in proportion to the total amount of ammonium ions and ammonia molecules, while the respective proportion at the pH of 8.8 is already up to about 20%. Changing the pH of liquid manure is difficult because of a high buffer capacity contained in liquid manure. Raising the pH of raw liquid manure requires the use of a large amount of chemicals, making the process economically non-viable; 3) The removal of nitrogen involves a powerful odor problem as untreated liquid manure is passed into the tower and the odor travels along with the air flow; 4) In industry, stripping is generally effected by using a single tall tower (at least 7 m in height, USEPA 2000). A condition in this type of technology is that the supplied solution has its nitrogen as far as possible in the form of ammonia. With the separation of ammonia, the solution's pH decreases and the tower's capacity to separate nitrogen is falling. In industry, the solutions are often 100% ammonia. The implementation of a single tall tower incurs extra costs because of technical solutions called for by the tower's height, resulting in a technical design which is fairly expensive.